Hot-rolled steel with very high strength and method for production

ABSTRACT

Hot-rolled steels provide increased strength without degrading elongation or weldability. Substitutional elements are included in the steel composition to increase the propensity of the steel to form martensite after hot-rolling processes despite relatively low cooling rates encountered during the hot-rolling processes.

PRIORITY

This application claims priority to U.S. Provisional Application Ser.Nos. 62/465,527 filed Mar. 1, 2017, entitled “Hot-Rolled Steel with VeryHigh Strength and Method for Production,” the disclosure of which isincorporated by reference herein.

BACKGROUND

The present application relates to an improvement in hot-rolled steelproducts. Hot-rolled steels are produced by subjecting an ingot of apredetermined thickness to a series of rollers to progressively decreasethe thickness of the ingot. Throughout the rolling process, the steel ismaintained at a very high temperature that is generally above therecrystallization temperature; final reduction passes may occur attemperatures below the recrystallization temperature of austenite. Oncethe rolling process is complete, the steel is coiled as it is cooling.The final steel coil is then cooled to ambient temperature.

In some circumstances, it can be desirable to increase the strength ofsteel materials used in hot-rolling processes. For instance, hot-rolledsteels can be used in the context of automotive frames. However, theautomotive industry continually seeks more cost-effective materials thatare lighter for more fuel-efficient vehicles. While thinner steelmaterials can meet this need, higher strength is necessary toaccommodate these thickness reductions. Thus, it is desirable toincrease the strength of steel materials used in hot-rolling processes.

SUMMARY

The steels of the present application solve the problem of poorweldability and low elongation in hot-rolled steels by a novel alloyingstrategy that incorporates transition metal elements that increase thepropensity of martensite formation after hot-rolling processes despiterelatively low cooling rates encountered during the hot-rollingprocesses.

DESCRIPTION OF DRAWINGS

FIG. 1 depicts a photomicrograph corresponding to composition reference4339-1 listed in Table 1.

FIG. 2 depicts a photomicrograph corresponding to composition reference4339-2 listed in Table 1.

FIG. 3 depicts a photomicrograph corresponding to composition reference4340-1 listed in Table 1.

FIG. 4 depicts a photomicrograph corresponding to composition reference4340-2 listed in Table 1.

FIG. 5 depicts a photomicrograph corresponding to composition reference4341-1 listed in Table 1.

FIG. 6 depicts a photomicrograph corresponding to composition reference4341-2 listed in Table 1.

FIG. 7 depicts a photomicrograph corresponding to composition reference4342-1 listed in Table 1.

FIG. 8 depicts a photomicrograph corresponding to composition reference4342-2 listed in Table 1.

DETAILED DESCRIPTION

The present embodiment involves a high strength, hot-rolled steel thatexhibits an ultimate tensile strength of approximately 1500 MPa.Although the steel of the present example is produced in a relativelyheavy gauge, or high thickness, of greater than 3 mm, it should beunderstood that in other embodiments various other suitable thicknessesmay be used.

As described above, the present embodiment exhibits generally highstrength. To achieve this high strength, the steel of the presentexample includes a predominately martensitic microstructure afterhot-rolling, coiling, and cooling to ambient temperature. To achievethis martensitic microstructure, the steel of the present embodiment hassufficient hardenability or susceptibility to thermal heat treatment.The term “sufficient hardenability” is defined by the formation ofmartensite during coiling and after hot rolling.

It should be understood that martensite is generally more likely to formin response to relatively fast cooling rates. However, in the presentembodiment the hardenability of the steel is sufficiently high such thatmartensite forms even with the relatively slow cooling rates that arepresent in commercial hot-rolling and coiling operations.

Carbon is generally understood to have a direct relationship withhardenability. In other words, increasing carbon additions to a steelcan likewise increase hardenability. However, in some circumstances itmay be undesirable to rely exclusively on carbon content to obtaindesired hardenability. For instance, when carbon additions exceedcertain levels, the weldability and the elongation to fracture of thesteel can be reduced. In the present embodiment, these detrimentalcharacteristics are avoided while also increasing hardenability of thesteel through use of substitutional or transition metal elements in lieuof increasing carbon substantially. By way of example only, thesesubstitutional or transition metal elements can include manganese,molybdenum, niobium, vanadium, chromium, or some combination thereof.

In embodiments of the present alloys, manganese is the primary alloyingaddition used to increase hardenability of the steel while avoidingother detrimental conditions such as reduced weldability and reducedelongation to fracture. Other elements such as molybdenum, niobium,chromium, and/or vanadium can also be similarly used to increasehardenability.

In the present embodiment, carbon is held at a relatively low level thatwill be described in greater detail below. Meanwhile, as describedabove, certain substitutional or transition metal elements are added toincrease hardenability. The particular amount of increased hardenabilityis determined by the increase required to promote the formation ofmartensite despite the relatively slow cooling rates encountered duringcoiling and subsequent ambient air cooling. In some embodiments, thecooling rate can be approximately 0.05 to 2° C./s. Of course, in otherembodiments different cooling rates can be used while still promotingthe formation of martensite.

In addition to iron and other impurities incidental to steelmaking, theembodiments of the present alloys include manganese, silicon, chromium,molybdenum, niobium, vanadium, and carbon additions in concentrationssufficient to obtain one or more of the above benefits. The effects ofthese and other alloying elements are summarized as:

Carbon is added to reduce the martensite start temperature, providesolid solution strengthening, and to increase the hardenability of thesteel. Carbon is an austenite stabilizer. In certain embodiments, carboncan be present in concentrations of 0.1-0.50 weight %; in otherembodiments, carbon can be present in concentrations of 0.1-0.35 weight%. In still other embodiments, carbon can be present in concentrationsof about 0.22-0.25 weight %.

Manganese is added to reduce the martensite start temperature, providesolid solution strengthening, and to increase the hardenability of thesteel. Manganese is an austenite stabilizer. In certain embodiments,manganese can be present in concentrations of 3.0-8.0 weight %; in otherembodiments, manganese can be present in concentrations of 2.0-5.0weight %; in still other embodiments, manganese can be present inconcentrations greater than 3.0 weight %-8.0 weight %; and in stillother embodiments, manganese can be present in concentrations greaterthan 3.0 weight %-5.0 weight %.

Silicon is added to provide solid solution strengthening. Silicon is aferrite stabilizer. In certain embodiments, silicon can be present inconcentrations of 0.1-0.5 weight %; in other embodiments, silicon can bepresent in concentrations of 0.2-0.3 weight %.

Molybdenum is added to provide solid solution strengthening, to increasethe hardenability of the steel, and to protect against embrittlement. Incertain embodiments, molybdenum can be present in concentrations of0-2.0 weight %; in other embodiments, molybdenum can be present inconcentrations of 0-0.6 weight %; in still other embodiments, molybdenumcan be present in concentrations of 0.1-2.0 weight %; in otherembodiments, molybdenum can be present in concentrations of 0.1-0.6weight %; in yet other embodiments molybdenum can be present inconcentrations of 0.4-0.5 weight %; and in yet other embodimentsmolybdenum can be present in concentrations of 0.3-0.5 weight %.

Chromium can be added to reduce the martensite start temperature,provide solid solution strengthening, and increase the hardenability ofthe steel. Chromium is a ferrite stabilizer. In certain embodiments,chromium can be present in concentrations of 0-6.0 weight %; in otherembodiments, chromium can be present in concentrations of 2.0-6.0 weight%; in other embodiments, chromium can be present in concentrations of0.2-6.0 weight %; and in other embodiments chromium can be present inconcentrations of 0.2-3.0 weight %.

Niobium can be added to increase strength and improve hardenability ofthe steel. In some embodiments niobium can also be added to provideimproved grain refinement. In certain embodiments, niobium can bepresent in concentrations of 0-0.1 weight %; in other embodiments,niobium can be present in concentrations of 0.01-0.1 weight %; and inother embodiments, niobium can be present in concentrations of0.001-0.055 weight %.

Vanadium can be added to increase strength and improve hardenability ofthe steel. In certain embodiments, vanadium can be present inconcentrations of 0-0.15 weight %; and in other embodiments, vanadiumcan be present in concentrations of 0.01-0.15 weight %.

Boron can be added to increase the hardenability of the steel. Incertain embodiments, boron can be present in concentrations of 0-0.005weight %.

The hot-rolled steels can be processed using conventional steel making,roughing, and finishing processes. For example, the steels can becontinuously cast to produce slabs of approximately 12-15 cm inthickness. Slabs are then reheated at temperatures of 1200-1320° C., andhot-rolled to a final gauge of ≥2.5 mm, with the final reduction passoccurring at a temperature of approximately 950° C. Scale on thehot-rolled steel coil can be removed by pickling and/or abrasiveblasting using processes that are known in the art.

The alloys of the present application can be as-hot-rolled (that is,bare or uncoated) or they can also be coated with an aluminum-basedcoating, a zinc-based coating (either galvanized or galvannealed), afterhot-rolling and scale removal. Such coating can be applied to the steelsheet using processes known in the art, including hot dip coating orelectrolytic coating.

Example 1

Various steel samples were prepared with the compositions shown below inTable 1. Generally, carbon was held at a fixed concentration. Meanwhile,the concentration of various substitutional or transition metal elementswas varied while carbon remained constant to test the impact of theseelements. These elements included manganese, chromium, molybdenum,and/or niobium.

TABLE 1 Composition range. Compositions are in weight percent. ReferenceC Si N Mn Cr Cu Ni P S Ti Al Mo Nb V 4339-1 0.228 0.26 0.0032 1.98 0.200.000 0.000 0.003 0.004 0.001 0.003 0.002 0.001 0.001 4339-2 0.223 0.260.0041 2.98 0.20 0.001 0.000 0.003 0.004 0.001 0.003 0.001 0.001 0.0014340-1 0.231 0.25 0.0053 3.99 0.20 0.000 0.000 0.002 0.007 0.001 0.0030.001 0.002 0.001 4340-2 0.230 0.25 0.0060 4.97 0.20 0.000 0.000 0.0020.008 0.001 0.003 0.001 0.001 0.010 4341-1 0.228 0.25 0.0066 3.96 0.200.001 0.000 0.006 0.008 0.002 0.003 0.480 0.051 0.001 4341-2 0.224 0.260.0089 3.97 0.20 0.000 0.001 0.006 0.009 0.002 0.004 0.480 0.051 0.0964342-1 0.229 0.25 0.0100 3.00 2.98 0.001 0.001 0.002 0.008 0.002 0.0030.002 0.002 0.001 4342-2 0.233 0.25 0.0072 2.97 2.92 0.001 0.001 0.0070.008 0.002 0.003 0.480 0.055 0.001

Example 2

Ingots were formed for each composition described above in Table 1. Theingots were formed by vacuum melting each composition in an inductionfurnace to cast 11-kg ingots. The as-cast ingots had an initialthickness of 45 mm. Once formed, the ingots were reheated to 1316° C.and rolled to a final thickness of approximately 3.6 mm. The rolling ofeach ingot was completed in eight passes. On the final rolling pass, atemperature measurement was taken and it was observed that thetemperature of each ingot was <955° C. After rolling, coiling wassimulated by subjecting each ingot to furnace equilibration atapproximately 566° C. with a range of 450 to 650° C. and subsequentcooling to ambient temperature.

Example 3

After the ingots were subjected to the simulated rolling and coilingprocesses described above in Example 2, micrographs were prepared usinga Nital etch. FIG. 1 shows a micrograph of an ingot with the compositionof reference 4339-1 in Table 1. FIG. 2 shows a micrograph of an ingotwith the composition of reference 4339-2 in Table 1. FIG. 3 shows amicrograph of an ingot with the composition of reference 4340-1 inTable 1. FIG. 4 shows a micrograph of an ingot with the composition ofreference 4340-2 in Table 1. FIG. 5 shows a micrograph of an ingot withthe composition of reference 4341-1 in Table 1. FIG. 6 shows amicrograph of an ingot with the composition of reference 4341-2 inTable 1. FIG. 7 shows a micrograph of an ingot with the composition ofreference 4342-1 in Table 1. FIG. 8 shows a micrograph of an ingot withthe composition of reference 4342-2 in Table 1.

Example 4

Ingots made with compositions of references 4339-1, 4339-2, and 4340-1were observed to include varying amounts of ferrite, pearlite, andbainite. A martensitic microstructure was observed in ingots made withcompositions of references 4340-2, 4341-1, 4341-2, 4342-1, and 4342-2.The presence of martensite in these samples was unexpected whenconsidering the cooling rates applied to each ingot. As described above,relatively slow cooling rates generally favor the formation of ferrite,pearlite, and bainite over the formation of martensite. However,martensite formation was observed even though the expectation wasferrite, pearlite, bainite, and/or other non-martensitic constituents.

Based on the observations above, it was found that a martensiticmicrostructure can be formed when manganese is at least 5 wt. % whileother substitutional elements are minimal and the carbon content isapproximately 0.23 weight %. Less manganese can be present while stillforming a martensitic microstructure if other substitutional elementsare included. For instance, for steels containing approximately 4 wt. %manganese, additions of molybdenum, niobium, and/or vanadium can stillpromote the formation of a martensitic microstructure. Similarly, forsteels containing approximately 3 wt. % manganese, an addition of 3 wt %chromium can still promote the formation of a martensiticmicrostructure.

Example 5

After the ingots were subjected to the simulated rolling and coilingprocesses discussed above in Example 2, mechanical testing was alsoperformed. Table 2, shown below, provides the results of the mechanicaltesting for each composition provided in Table 1.

TABLE 2 Chemical composition of certain embodiments of the presentalloys Yield Strength Ultimate Tensile Total Elongation Reference (Mpa)Strength (MPa) (%) 4339-1 429 608 23.2 4339-2 599 845 13.0 4340-1 7571241 11.9 4340-2 873 1488 9.0 4341-1 998 1444 9.6 4341-2 985 1417 9.24342-1 988 1517 8.1 4342-2 1024 1568 9.6

As can be seen in Table 2, the compositions noted above in Example 4 asbeing susceptible to formation of martensitic microstructure afterhot-rolling and relatively slow cooling also exhibited tensile strengthsof approximately 1500 MPa. Ultimate tensile strengths in excess of 1400MPa were achieved using several alloy strategies that producedmartensitic microstructure in the as-hot-rolled condition. As describedabove in Example 4, this could include alloying with only manganese(e.g., reference 4340-2), alloying with a combination of manganese,molybdenum, and niobium (e.g., reference 4341-1), alloying with acombination of manganese, molybdenum, niobium, and vanadium (e.g.,reference 4341-2), alloying with a combination of manganese, andchromium (e.g., reference 4342-1), and alloying with a combination ofmanganese chromium, molybdenum, and niobium (e.g., reference 4342-2).

For the compositions noted above as producing martensite in theas-hot-rolled condition, it was expected for the martensite to provide ahard and strong steel. The data provided above in Table 2 confirms thatthe martensite containing steels were strong with tensile strengths ofapproximately 1500 MPa. However, unexpectedly, the martensite containingsteels exhibited relatively high elongation given the expected hardnessof the steels. As can be seen above, total elongation was approximately8-10%.

Example 6

A high strength steel comprising by total weight percentage of thesteel:

(a) from 0.1% to 0.5%, preferably from 0.1% to 0.35%, more preferablyfrom 0.22-0.25%, Carbon;

(b) from 2.0% to 8.0%, preferably from greater than 3.0% to 8%; morepreferably from 2.0 to 5.0%, and more preferably from greater than 3.0%to 5.0%, Manganese; and

(c) from 0.1% to 0.5%, preferably from 0.2% to 0.3%, Silicon.

Example 7

A high strength steel of Example 6 or any one of the following Examples,further comprising from 0.0% to 6.0%, preferably from 0.0% to 2.0%, morepreferably 0.1% to 6.0%, more preferably 0.1% to 2.0%, more preferably0.1% to 0.6%, and more preferably 0.4% to 0.5%, Molybdenum.

Example 8

A high strength steel of either one of Examples 6 and 7, or any one ofthe following Examples, further comprising from 0% to 6.0%, preferably0.2% to 6.0%, more preferably 2.0% to 6.0%, and more preferably 0.2% to3.0%, Chromium.

Example 9

A high strength steel of any one of Examples 6 through 8, or any one ofthe following Examples, further comprising from 0.0% to 0.1%, preferably0.01% to 0.1%, more preferably 0.001 to 0.055% Niobium.

Example 10

A high strength steel of any one Examples 6 through 9, or any one of thefollowing Examples, further comprising from 0.0% to 0.15%, preferably0.01% to 0.15%, Vanadium.

Example 11

A high strength steel of any Examples 6 through 10, or any one of thefollowing Examples, further comprising from 0% to 0.005% Boron.

Example 12

A high strength steel of any one of Examples 6 through 11, or any one ofthe following Examples, wherein the steel has, after hot-rolling andcoiling, an ultimate tensile strength of at least 1480 MPa and a totalelongation of at least 6%.

Example 13

A high strength steel of any one of Examples 6 through 12, or any one ofthe following Examples, wherein the steel has, after hot-rolling andcoiling, an ultimate tensile strength of approximately 1500 MPa and atotal elongation of approximately 8 to 10%.

Example 14

A high strength steel of any one of Examples 6 through 13, wherein thesteel is coated with an aluminum-based coating or a zinc-based coating(either galvanized or galvannealed), after cold rolling and before hotstamping.

What is claimed is:
 1. A high strength steel, the steel comprising: 0.1to 0.5% carbon; 2.0 to 8.0%, manganese; 0 to 2.0% molybdenum; 0 to 0.1%niobium 0 to 0.15% vanadium 0 to 6.0% chromium; and the balanceincluding iron and impurities.
 2. The steel of claim 1, wherein theconcentration of manganese comprises 3.0 to 4.0%.
 3. The steel of claim2, wherein the concentration of molybdenum comprises 0.1 to 0.6%,wherein the concentration of niobium comprises 0.01 to 0.1%.
 4. Thesteel of claim 3, wherein the concentration of vanadium comprises 0.001to 0.096%.
 5. The steel of claim 1, wherein the steel has an ultimatetensile strength of approximately 1500 MPa and a total elongation ofapproximately 8 to 10%.
 6. The steel of claim 1, wherein theconcentration of manganese, molybdenum, niobium, and vanadium isconfigured to increase hardenability of the steel to a predeterminedlevel, wherein the predetermined level of hardenability is sufficient toform martensite in response to slow cooling during coiling of the steelafter hot-rolling.
 7. The steel of claim 1, having two outer surfaces,and further comprising an aluminum-based coating or a zinc-based coatingapplied to at least one outer surface.
 8. A high strength steel, thesteel comprising, by weight percent, 0.01 to 0.5% carbon, 2.0 to 8.0%manganese, 0.1 to 0.5% silicon, and at least one of 0.1 to 2.0%molybdenum, 0.2 to 6.0% chromium, 0.01 to 0.1% niobium, and 0.01 to0.15% vanadium.
 9. The steel of claim 8, further comprising 0.1 to 0.35%carbon.
 10. The steel of claim 8, further comprising 3.0 to 8.0%manganese.
 11. The steel of claim 8, further comprising 2.0 to 5.0%manganese.
 12. The steel of claim 8, having two outer surfaces, andfurther comprising an aluminum-based coating or a zinc-based coatingapplied to at least one outer surface.